The invention pertains to an extrusion blow-molding method and to a device for performing an extrusion blow-molding method.
In an extrusion blow-molding method, thermoplastic material is plasticized in an extruder to form a melt and conveyed under pressure to a head. The parison leaves the outlet of the head continuously at a certain delivery speed. The blow mold, usually located underneath the head, receives the parison, which is in the shape of a tube hanging from the head. The tube is expanded in the blow mold to obtain the finished hollow body. After extrusion of a given length of the parison, a cutting device, for example, is used to separate the parison underneath the outlet of the head. A separated parison can also be transferred to the blow mold by means of a handling unit.
For the production of satisfactory hollow bodies such as bottles or canisters, it is necessary for the parison leaving the head to comprise a minimum length. At the same time, the parison may not exceed a certain length, because otherwise more material than necessary is consumed. Another reason why it is necessary to maintain the correct length of the parison is that, in modern extrusion blow-molding methods, the wall thickness of the parison is often controlled automatically by a predefined program.
During the extrusion blow-molding method, fluctuations occur in the viscosity, in the conveying rate, in the regenerate, in the material, and in the bulk density. As a result of these fluctuations, it is impossible to prevent parisons of different lengths from being extruded during the various blow-molding cycles. If the length exceeds the nominal length, too much waste is generated. If the length falls below the nominal length, it is possible that rejects will be produced. There is also the danger that, when the wall thickness is controlled automatically by a parison programming device, the areas with certain wall thicknesses will not be in their correct positions relative to the blow mold receiving the extruded parison.
It is known from DE 25 44 171 A that a light barrier can be set up underneath the blow mold; as soon as the extruded tubular parison has reached the nominal length, its bottom end interrupts this barrier. If the time required to reach the nominal length is less than the nominal time in which, if the extruder is operating with absolute regularity, the tubular parison achieves its nominal length, the extruder is operating too fast. In this case, a control unit decreases the rotational speed of the screw. If the time needed to achieve the nominal length exceeds the nominal time, the extruder is operating too slowly. In this case, the control unit increases the rotational speed of the screw of the extruder, insofar as the amount by which the nominal time is exceeded lies outside the planned tolerance range. At the time the parison is detected by the light barrier, it then has a length corresponding to the nominal length. This makes it possible to prevent rejects from occurring as a result of parisons which are too short. Changes in the extrusion rate by adjustments to the screw rotational speed, however, do not go into effect until the next blow-molding cycle. This known device therefore does not make it possible to avoid fluctuations in the extrusion rate and thus in the time available to extrude a tubular parison. Considerable waiting times within a blow-molding cycle are therefore required to carry out the extrusion blow-molding method.
It is known from DE 25 44 609 A, furthermore, that the gap width of the outlet of the head can be adjusted as a function of changes in the length of the parison.
A method of the class in question for the blow-molding of hollow bodies is known from DE 10 2004 015 719 B4. The method comprises an automatic control . . . pointed out, which corrects the extrusion rate in such a way that the parison assumes a previously determined position in the blow mold. The automatic control method is based on a wall thickness program, which acts on the width of the die gap in the critical cross-sectional areas, wherein the automatic control circuit compares the actual weight of a proportionally divided bottom waste slug and/or of the proportionally divided hollow body with a corresponding nominal weight value. The position of the following parison is then adjusted as a function of the nominal-actual weight value comparison through a change in the width of the die gap and/or of the rpm's of the extruder screw.
To prevent the parisons from sticking or welding together after a parison has been cut off, it is proposed in EP 1 354 692 A1 that the delivery of the parison from the die of the die head be briefly interrupted during the process of separating the individual parisons from each other. To interrupt this delivery process, the melt is preferably diverted from the extruder into a melt accumulator. The melt being stored temporarily during the continuous extrusion process can be added back into the melt being fed to the die head again after the interruption. The melt accumulator is controlled on the basis of flow resistance, which is determined by the distance to the accumulation space of the melt accumulator and the distance to the die head.